As flame retardant fibers, there are known acrylic fibers and polyester fibers in which flame retardant monomers are copolymerized, rayon fibers in which a flame retardant is kneaded or reacted, thermosetting fibers or aramid fibers whose polymers themselves are flame retardant, cotton or wool that are post-processed with a flame retardant, and the like. However, in acrylic fibers hydrogen cyanide gas is produced when they are burned. Polyester fibers are melt-dripped. Thermosetting fibers are low in fiber strength. Aramid fibers are expensive. Cotton and wool have problems such as texture-hardening by post-processing, low durability against washing, and the like. Studies have been made for improvement in the respective fibers.
On the other hand, PVA-based flame retardant fibers are known in, for example, Japanese Patent Application Publication Nos. 37-12920 and 49-10823. They are used in clothes such as uniforms for fire fighters and working clothes, living materials such as carpets, industrial materials such as car seats, and the like. However, they are expensive. In the present situation, quantitative expansion is difficult.
Conventional PVA-based flame retardant fibers are fibers obtained by adding an emulsion of vinyl-chloride-based polymer (abbreviated to PVC hereinafter)/water to an aqueous PVA solution and then spinning the resultant dope. PVC, however, is water-insoluble. Consequently, in the conventional method of using water as a dope solvent, it is impossible to use powdery PVC, which is commercially available, low-priced PVC. Thus, PVC emulsion, which is several times as expensive as the powdery PVC, is used. In order to make PVA-based fibers flame retardant, this expensive PVC must be used in an amount of several ten percents of PVA, resulting in high cost of the PVA-based fibers. A mixed aqueous solution of PVA and PVC emulsion is not stable at 70-100.degree. C. near spinning temperature, and is especially insufficient in mechanical stability when the solution passes through a gear pump. For stabilization, therefore, it is necessary to add a surfactant thereto. This causes higher cost.
Conventional PVA-based flame retardant fibers are produced by mixing PVC emulsion having an emulsion particle size of 0.01-0.08 .mu.m with an aqueous PVA solution; if necessary, adding thereto a tin compound or an antimony compound to obtain a dope; wet spinning the dope into a solidifying bath comprising an aqueous solution of sodium sulfate; subjecting the resultant to drying, dry heat drawing and thermal treatment; and, if necessary, acetalizing the resultant by formalin for improving hot water resistance. Moreover, in order to make its strength higher, the following method is also performed: a dope wherein boric acid is added to a mixed aqueous solution of PVA and PVC emulsion is extruded into a solidifying bath comprising a mixed aqueous solution of sodium hydroxide and sodium sulfate, and then the resultant is subjected to a boric acid-crosslinking process. In any one of these processes, however, because of use of sodium sulfate, which is a dehydrating salt, as the content in a solidifying bath, fine skin layers are formed on the surface of the fiber immediately after solidification. As a result, its section becomes a non-uniform skin/core structure. In its core portion, crystallization is liable to become insufficient. In fact, the crystallinity degree of PVA of this fiber is a small value of 50-60%. Accordingly, there remains room for improving dimension stability, especially dimension stability between dry and wet states even if the fiber is subjected to formalization.
As described above, although conventional PVA-based flame retardant fibers have excellent points compared with other flame retardant fibers, the use thereof is limited because their manufacturing costs are high and their dimension stability is insufficient.
An object of the present invention is to provide a PVA flame retardant fiber which can be industrially produced at low costs and is excellent in spinning stability, and to overcome the drawback that conventional PVA flame retardant fibers are poor in dimension stability in hot water.